This invention relates generally to a method and apparatus for transferring heat in a heat exchanger, more particularly, to an electrode arrangement for optimizing electrohydrodynamic (EHD) enhancement of heat and mass transfer.
It has been well known in the art that electric fields can have an effect upon heat transfer. It is further known that the rate at which liquid at one temperature vaporizes when in contact with a surface at a higher temperature can be enhanced by locating that surface within the electric field generated by an electrode, insulated from the surface and connected to a source of high voltage. Despite this knowledge, commercial use of such phenomenon, to which the general description electrohydrodynamic (EHD) enhancement has been applied, has been limited.
In order to effectively facilitate EHD enhancement of heat transfer, it is necessary to create an electric field that is as strongly inhomogeneous as possible. More importantly, the regions of the electric field having the strongest electric field gradients should be as close as possible to the heat transfer surface. Given these guidelines, the electrode designs that have been described in the literature appear to be limited in performance. The typical design uses a metal pipe as one electrode and a parallel system of wires as the second electrode. While the field around the small diameter wires can be highly inhomogeneous, the field around the large diameter pipe tends to be more uniform. Indeed the presence of surface enhancements such as low fins seems to be necessary for good performance in this general arrangement. Since the pipe is the actual heat transfer surface, it seems to be particularly disadvantageous that the strongest gradient is near the wires not near the pipe. Agitation of bubbles and droplets near the pipe and disruption of the surrounding thermal boundary layers should be the aim of the design with the goal of enhancing two phase heat transfer to or from the pipe.
Another design is disclosed in the patent of Allen et al., U.S. Pat. No. 4,651,806, issued Mar. 24, 1987. Allen et al. disclose a xe2x80x9cshell-tubexe2x80x9d type heat exchanger that employs EHD technology. The device of Allen et al. has a plurality of spaced-apart heat exchange tubes that pass through a casing. In addition, an electrode is located within the casing but insulated from both the casing and the tubes. Heat exchange takes place through the tube walls between a first fluid medium within the tubes and a second medium outside the tubes but within the casing when the electrode is excited to high voltage. However, because the electrode is insulated from the tubes (the heat transfer surface), the strongest electric field gradients do not reside near the heat transfer surface. Thus, the resulting heat transfer due to EHD effect is not optimized.
While apparatuses have employed EHD effects to enhance the rate of heat transfer, it appears that none have employed a commercially feasible electrode arrangement to optimize heat and mass transfer from a heat transfer surface to a dielectric fluid. In addition, such apparatuses are fragile, therefore requiring frequent maintenance, and are difficult to manufacture.
What is needed, therefore, in the art, is an electrode arrangement for electrohydrodynamic (EHD) enhancement of heat and mass transfer in dielectric fluids that will place the regions of strongest electric field gradient at or near the heat transfer surface. Moreover, what is needed but unavailable in the art is a rugged electrode arrangement that will provide for simultaneous heat and mass transfer enhancement on both sides of the heat transfer surface.
EHD enhancement of heat and mass transfer utilizes an electric field to influence fluid flows or related transport effects, such as bubble growth or departure, that augment heat and mass transfer in a dielectric fluid. While the interaction of an electric field with a fluid is difficult to predict or even interpret, it appears that the dielectrophoretic force predominates in an uncharged, nonconductive fluid. The dielectrophoretic force requires a gradient in the electric field and a nonuniform electric permittivity. A prime example of nonuniform permittivity is a droplet in a vapor medium, The droplet is a fluid lump of higher electric permittivity immersed in a vapor medium of relatively lower permittivity. In this situation, the dielectrophoretic force tends to drive the droplet toward a region of higher electric field strength. In contrast, a vapor bubble (a region of lower permittivity) immersed in a liquid (of higher electric permittivity) will be driven to a region where the electric field strength is lower. The strength of the dielectrophoretic force on a spherical droplet or bubble is roughly proportional to two factors, the difference in permittivity between the two phases and the gradient of the square of the electric field strength.
The most effective EHD technology should create an electric field that is as strongly inhomogeneous as possible. In addition, the field should be oriented so that phenomena that enhance heat transfer are stimulated. In an evaporator for example, vapor bubbles should be forced away from the surface and liquid should be attracted. It is therefore axiomatic that the strongest electric field gradients should reside as close as possible to the heat transfer surface.
In the present invention, this advantageous result is achieved by embedding a set of electrodes within the heat transfer wall material. Heat transfer wall material having interior and exterior heat transfer surfaces is typically attached to a tube sheet, fitting or other confinement typically used in refrigeration evaporators, evaporative condensers, cooling towers, and other evaporators and boilers. The heat transfer wall material of the present invention is generally in the shape of a tube or plate, but can also be formed to have other geometric characteristics.
In a first embodiment of the present invention, a set of electrodes is embedded within the heat transfer wall material between the exterior heat transfer surface and the interior heat transfer surface. A set of electrodes comprises one or more electrodes adapted to be excited to a large relative voltage with respect to another set of electrodes. One set of electrodes will be at a higher electrical potential relative to the other. The higher potential set will be referred to as the positive set, and the other set will be referred to as the negative set. Only the relative potential of the electrodes is of functional importance. For convenience or safety, one set can be near or at ground potential. For example, if the negative electrode is grounded, the positive electrode would be above ground potential. If the positive electrode is grounded, then the negative electrode would be below ground potential.
When the heat transfer wall material is tube-shaped, the set of electrodes forms helices which extend throughout the length of the wall material. The electrodes are arranged such that they are spaced apart and alternate in polarity. When the heat transfer wall material is plate-shaped, the set of electrodes are interdigitated and extend throughout the length of the wall material. Again, the electrodes are arranged such that they are spaced apart and alternate in polarity. In both the tube-shaped and plate configuration, the electrodes are connected to the high voltage source at either end of the heat transfer wall material, where the heat transfer wall material passes through a tube sheet (a bulkhead penetrated by heat exchanger tubes, plates or other heat transfer wall material), fitting, or other confinement. When more than one electrode of each polarity comprises the set of electrodes, the electrodes having the same polarity are ganged together, in any number of ways commonly known in the art, to facilitate connection to the high voltage source.
When a high positive and high negative voltage is supplied to the electrode set an electric field results. Electric field gradients are produced along the length of the interior heat transfer surface and the exterior heat transfer surface of the heat transfer wall material. The regions of intense electric field gradient reside closest to the heat transfer surfaces. When nonconductive fluids pass along both surfaces of the heat transfer wall material, the strong electric field gradients near the heat transfer surfaces repel vapor bubbles from these regions and attract the liquid. The result is augmented evaporation.
In a second embodiment of the present invention, two distinct sets of electrodes are embedded in the heat transfer wall material. Like the first embodiment, electrodes of alternating polarity extend throughout the length of the heat transfer wall material. Unlike the first embodiment, however, all of the electrodes are not simultaneously electrified. Instead, each set of electrodes is alternately and independently electrified. When the first set of electrodes is electrified, the second set of electrodes therebetween remain uncharged. When this occurs, the nonconductive fluid moves to the electric field gradients above the charged electrode set. When the second set is electrified, the first set therebetween remains uncharged and the nonconductive fluid is then attracted to the electric field gradients above the second charged electrode set. The resulting agitation produced by the wavelike motion of the fluid layer thus increases evaporation.
While this embodiment can be employed using the tube-shaped heat transfer wall material, it is particularly suited for heat transfer wall material in the form of plates for cooling tower applications. It should be evident to those skilled in the art that more than two distinct sets of electrodes can be used in this embodiment.
It is, therefore, an object to the invention to provide a method and apparatus for improving the heat and mass transfer rate in a heat exchanger.
A second object of the invention is to provide an electrode design for a heat exchanger that will result in augmented heat transfer, particularly evaporation.
Another object of the invention is to provide an electrode design for an electrically nonconducting heat transfer wall material which will compensate for the relatively low thermal conductivity of the wall material by enhancing heat transfer on both sides of the wall surface.
A further object of the invention is to provide an electrode design for a heat exchanger which will eliminate the need for fragile and complicated arrays of wire electrodes.
A still further object of the invention is to provide an electrode design for a heat exchanger whereby the electrodes are securely and safely embedded within the wall material, thereby allowing the high voltage electrical connections to be made at either end of the heat transfer wall material where the heat transfer wall material passes through a tube sheet, fitting, or other confinement.
An additional object of the present invention is to provide an apparatus for enhancing heat and mass transfer in dielectric fluids using EHD that is sturdy in construction and simple to manufacture.
These and other objects are attained by the present invention, which relates to an apparatus that utilizes electric field gradients to influence fluid flows and related transport effects, such as bubble growth or departure, that augment heat and mass transfer in a dielectric fluid. Electrodes of alternating polarity are buried in a dielectric heat transfer wall material. The regions of highest electric field gradient are near the heat transfer surfaces and provide for simultaneous augmentation of heat and mass transfer on both sides of the wall material. In an alternative embodiment, electrodes can be buried in a fill material for application in cooling towers. In either application but especially the latter, it is possible to provide two sets of electrodes that can be alternately electrified.
Additional objects and advantages of the invention will become apparent from the following detailed description, with reference to the accompanying drawings.